From 907593e2e192c857a039e023a3ce8ea668d364fe Mon Sep 17 00:00:00 2001
From: missing <piecenotfound@gmail.com>
Date: Wed, 14 Dec 2022 19:49:51 -0600
Subject: [PATCH] day 12

---
 src/days/day12.rs | 132 ++++++++++++++++++++++++++++++++++++++++++++++
 src/lib.rs        |   1 +
 2 files changed, 133 insertions(+)
 create mode 100644 src/days/day12.rs

diff --git a/src/days/day12.rs b/src/days/day12.rs
new file mode 100644
index 0000000..a95ae67
--- /dev/null
+++ b/src/days/day12.rs
@@ -0,0 +1,132 @@
+use std::{
+    cmp::{Ordering, Reverse},
+    collections::{BinaryHeap, HashMap},
+};
+
+use aoc_runner_derive::{aoc, aoc_generator};
+
+struct PuzzleInput {
+    heights: Vec<Vec<u8>>,
+    start: (usize, usize),
+    end: (usize, usize),
+}
+
+#[aoc_generator(day12)]
+fn generator(input: &str) -> PuzzleInput {
+    let mut start = None;
+    let mut end = None;
+
+    let nodes = input
+        .lines()
+        .enumerate()
+        .map(|(i, l)| {
+            l.chars()
+                .enumerate()
+                .map(|(j, c)| match c {
+                    'S' => {
+                        assert!(start.replace((i, j)).is_none());
+                        1
+                    }
+                    'E' => {
+                        assert!(end.replace((i, j)).is_none());
+                        26
+                    }
+                    'a'..='z' => c as u8 - b'a' + 1,
+                    _ => panic!(),
+                })
+                .collect()
+        })
+        .collect();
+
+    PuzzleInput {
+        heights: nodes,
+        start: start.unwrap(),
+        end: end.unwrap(),
+    }
+}
+
+fn neighbours(coord: (usize, usize), heights: &[Vec<u8>]) -> impl Iterator<Item = (usize, usize)> {
+    let current_height = heights[coord.0][coord.1];
+
+    let a = (coord.0 + 1, coord.1);
+    let a = heights
+        .get(a.0)
+        .and_then(|v| (v[a.1] >= current_height - 1).then_some(a));
+
+    let b = coord.0.checked_sub(1).map(|v| (v, coord.1));
+    let b = b.filter(|b| heights[b.0][b.1] >= current_height - 1);
+
+    let c = (coord.0, coord.1 + 1);
+    let c = heights[c.0]
+        .get(c.1)
+        .and_then(|v| (*v >= current_height - 1).then_some(c));
+
+    let d = coord.1.checked_sub(1).map(|v| (coord.0, v));
+    let d = d.filter(|d| heights[d.0][d.1] >= current_height - 1);
+
+    a.into_iter().chain(b).chain(c).chain(d)
+}
+
+struct CmpByFirst<T, U>(T, U);
+
+impl<T: PartialEq, U> PartialEq for CmpByFirst<T, U> {
+    fn eq(&self, other: &Self) -> bool {
+        self.0.eq(&other.0)
+    }
+}
+
+impl<T: Eq, U> Eq for CmpByFirst<T, U> {}
+
+impl<T: PartialOrd, U> PartialOrd for CmpByFirst<T, U> {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        self.0.partial_cmp(&other.0)
+    }
+}
+
+impl<T: Ord, U> Ord for CmpByFirst<T, U> {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.0.cmp(&other.0)
+    }
+}
+
+fn fill_dist_map(input: &PuzzleInput) -> HashMap<(usize, usize), u32> {
+    let mut to_visit = BinaryHeap::new();
+    to_visit.push(CmpByFirst(Reverse(0), input.end));
+
+    let mut map = HashMap::new();
+    map.insert(input.end, 0);
+
+    while let Some(CmpByFirst(Reverse(dist), coord)) = to_visit.pop() {
+        for neighbour in neighbours(coord, &input.heights) {
+            map.entry(neighbour)
+                .and_modify(|v| *v = (*v).min(dist + 1))
+                .or_insert_with(|| {
+                    to_visit.push(CmpByFirst(Reverse(dist + 1), neighbour));
+
+                    dist + 1
+                });
+        }
+    }
+
+    map
+}
+
+#[aoc(day12, part1)]
+fn part1(input: &PuzzleInput) -> u32 {
+    fill_dist_map(input)[&input.start]
+}
+
+#[aoc(day12, part2)]
+fn part2(input: &PuzzleInput) -> u32 {
+    let map = fill_dist_map(input);
+
+    input
+        .heights
+        .iter()
+        .enumerate()
+        .flat_map(|(i, v)| v.iter().enumerate().map(move |(j, v)| ((i, j), v)))
+        .filter_map(|(coord, h)| (*h == 1).then_some(coord))
+        .map(|coord| *map.get(&coord).unwrap_or(&u32::MAX))
+        .min()
+        .unwrap()
+}
diff --git a/src/lib.rs b/src/lib.rs
index 59c9d65..25e5eb4 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -13,6 +13,7 @@ mod days {
     // curse you, rustfmt
     mod day10;
     mod day11;
+    mod day12;
 }
 
 aoc_lib! { year = 2022 }